A system and method for determining the spatial position and the shape of a surgical instrument (1) having a deformable body (2) by: providing an elasticity model of the surgical instrument (1); defining at least one parameter which influences the shape of the instrument (1); determining the spatial position and/or orientation of at least one tracking sensor (3) of the surgical instrument (1); determining the value of the at least one parameter; calculating the position and/or orientation of at least one part of the surgical instrument (1) with the aid of the elasticity model together with the determined value of the at least one parameter and the determined spatial position and/or orientation of the at least one tracking sensor (3). The present invention also relates to a surgical instrument (1) having a deformable body (2) comprising at least two sensors (3, 4), wherein at least one sensor is a tracking sensor (3) and at least one other sensor is a sensor (4) which allows the value of a predetermined parameter influences the shape of the instrument (1) to be determined.